The measurement of power factor and power quality is an important aspect in the monitoring of electrical utilities. In order to calculate the power factor and quality of a particular AC power line, the alternating voltage and current of the line must be measured. For overhead power lines, removable contactless voltage measuring devices provide the best balance of ease of use and flexibility.
FIG. 1 illustrates a non-contact voltage measuring device 10 as disclosed in JP2002131341. The figure shows a longitudinal section of an insulated conducting wire 12, to which an alternating voltage is applied with respect to ground. This is the voltage waveform to be observed by the device 10. The device 10 consists of two conducting pipes—an inner conductor 14 and an outer conductor 16—that surround the wire 12. The two conducting layers of the device are usually separated by a dielectric layer 18. The output voltage of the device 10 may be picked up by a shielded cable (not shown) whose shield is connected to the outer conductor 16 and whose centre is attached to the inner conductor 14 through a small hole (not shown) in the outer conductor 16 and dielectric 18.
The arrangement shown in FIG. 1 is excellent for preventing interference pickup from stray electric fields created by other voltage sources external to the outer conductor of the device if the following two conditions are met:
1) The outer conductor 16 of the device 10 completely encloses the inner conductor 14 around its circumference.
2) The length of longitudinal overshoot L1 of the outer conductor 16 past the inner conductor 14 is greater than the diameter of the outer conductor 16.
Condition 2 makes sure that any ingress of interfering electric fields from the two ends of the device 10 is minimized.
When these two conditions are met, the outer conductor 16 of the device 10 effectively shields the inner conductor 14 from external electric fields and it responds only to electric fields due to the charge on the conductor 12.
However, it is apparent that the device shown in FIG. 1 can only be used in applications where the wire 12 can be threaded through the device or if the device has more than one moveable part that can be clipped or attached together.
Another restriction on the use of this type of device occurs if the wire diameter is large, as the length of the device becomes large in order to meet condition (2). For a wire of 3.5 cm diameter, condition (2) would result in a device typically 9 cm long.
Because of these restrictions, this low interference structure is virtually unused on high voltage overhead power lines.
The most common type of voltage measuring device for overhead power lines is shown in FIG. 2. Here a gap 20 is made in the dielectric 22 and the inner and outer conductors 24, 26 respectively. This allows the device to be easily attached to the existing power line. In practice, the length L of the device is also made much smaller than would be required to meet condition (2) above. Thus, the device shown in FIG. 2 suffers from interference from external fields entering through the gap 20 and the two ends of the device. The result is that the device is not usable in many applications for accurate power factor and quality measurements, particularly if an interfering phase is situated in the direction of the gap 20 in the device.
It is an object of the invention to provide an improved alternating voltage measuring device in which these disadvantages are avoided or mitigated.